Tunable selector circuits



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INVENTOR NOEL M. RUST ATTORNEY (4 N. M. RUST 2,071,902

TUNABLE SELECTOR CIRCUITS Feb. 23, 1937.

Filed June 26, 1933 7 Sheets-Sheet 5 l,lNvENToR NOEL M RUST ATTORNEY C|l ||i l A ATTORNEY N. RUST TUNABLE SELECTOR CIRCUITS Flled June 26,1933 l, l l l o l l l l l L llll'" Feb. 23, 1937.

Feb. 23, 1937. N. M. RUST 4 TUNABLE SELECTOR CIRCUITS 7 Sheets-Sheet 7Filed June 26,- 1935 I I I I I I I I I I I I I I I I I I I I I I I I I lI I I l I I I l I I I l I I |NvENToR` J| NOEL M.' RUST A-rrQRNEY Ar trieTUNABLE SELECTOR CIRCUITS Nol Meyer litust, Chelmsford, England,assigner to Radio Corporation of America, a corporation of DelawareApplication .lune 26, 1933, Serial No. 677,617 In Great Britain July 14,1932 14 Claims.

This invention relates to frequency selective circuit arrangements foruse in radio reception, and has for its main object to provide a circuitarrangement suitable for use for receiving broadcast radio reception andwhereby a high degree of selectivity may be obtained.

A further object of the invention is to obtain the required selectivityin' such manner that the degree or kind of tone correction necessary torcompensate for distortion due to the shape of the selectivity curveshall not be diiiicult to obtain practically.

It is well known that with the large number of broadcast transmittingstations in existence at the present time, it is a very difficult matterto obtain really selective reception in many places, and while fairlysatisfactory results have been obtained, for example by employing in areceiver a number of tuned circuits in cascade, or a number of band passtuning circuits in cascade, the degree of selectivity obtained is not ashigh as is desirable, and where very sharp selectivity is obtained theshape of the selectivity curve is often such as to make the subsequenttone correction i. ,5 necessary to compensate for side band distortiondue to that shape a very difficult matter to obtain practically.Further, known arrangements capable of producing reasonably satisfactoryresults both as regards selectivity and quality are for the most partexpensive and difficult to manufacture.

According to the first feature of this invention frequency selectivityis obtained by means of one or more branched circuits, each branch ofwhich consists of inductance and capacity in series and negativeresistance means is associated with said branched circuit for reducingthe effective damping thereof. It is well known that a circuitconsisting of two parallel branches each consisting of an inductance inseries with a condenser possesses three critical frequencies at the twoouter of which the branched circuit approximates to infinite susceptancewhile at a frequency between these two outer frequencies the 45 branchedcircuit approximates, to a condition of Zero susceptance.

In carrying out this feature of the present inventicn the branchedvcircuit is so arranged that the frequencies at which approximatelyinfinite susceptance is obtained correspond to the limiting frequenciesof a band of frequencies desired to be received, and preferably thearrangement is such that the intermediate frequency at whichapproximately Zero susceptance is obtained is 55 mid-Way between theouter frequencies.

According to a second or modified feature of this invention there areemployed what may be termed split or branched complex resonant circuitsinr cascade rather than in juxtaposition and the resonant frequencies ofthe separate complex circuitsbeing arranged slightly differently i. e.staggered so that each separate complex circuit gives an asymmetricalresponse curve, the two response curves together, however, resulting ina substantially symmetrical response curve not presenting thedisadvantage of a rising response on either side of a desired acceptanceband.

In the drawings,

Figs. l, 2 and 3 are sets of graphs provided for explanatory purposes,

Fig. 4 diagrammatically shows an arrangement in accordance with theinvention,

Figs. 5, 6, 'lare further sets of explanatory graphs,

Fig. 8 is a diagrammatic representation of a further embodiment inaccordance with the invention,

Figs. 9, 10, 1l, 12, 14, 15 illustrate diagrammatically various modiedarrangements incorporating the invention, while Fig. 13 is a set ofexplanatory graphs.

In order that the first feature of this invention may be the betterunderstood, the nature of the reactance and susceptance changes takingplace in a branched circuit consisting of two branches in parallel eachhaving an inductance and a condenser in series will now be described.Suppose the outermost frequencies are F1, F2 respectively, and theintermediate frequency is Fo and suppose also that the intermediatefrequency is the geometric mean of F1 and F2, a condition which involvesthat there shall be no mutual inductance between the inductances in thetwo branches of the circuit. Then Suppose the induct'ance and capacityin one branch are given by L1 C1 respectively, this branch beingresonant at frequency F1 and the inductance and capacity of the secondbranch which is resonant at the frequency F2, are L2 C2 respectively.Ignore for the moment the effect of resistance in the branches.

The reactance occurring at different frequencies for the two branchesare asshown in Fig. 1 in which the ordinates are values of reactance andthe abscissae values of frequency. This said Fig. 1 is drawn for thecase in which L1=L2 and C1 C2, Fo being equal to In Fig. 1 the curve IAis the reactance curve of the coil L1 which, of course, is equal to thereactance curve of the coil L2; the curve IB is the curve of reactanceof the condenser C1; the curve IC is the curve of the reactance of thecondenser C2; the curve ID the curve of total reactance of L1 and C1 andthe curve IE the curve of the total reactance of the combination L2 andC2. It will be seen that at the frequencies F1 and F2 one or otherbranch is of Zero reactance. The total reactance of the whole circuit isshown by the curve IF and as will be seen the total reactance is zero atF1 and F2 and infinity at Fo.

The foregoing conditions ignore the effect of resistance losses, and, ifit were not for such losses, it would be possible to construct a circuitwhich would produce sharp rejector effects for the frequencies F1 and F2and a sharp acceptor effect for the frequency F0. In practice, however,owing to resistance losses in the coils, as the frequencies F1 and F2are brought very near to each other by making the condensers C1 and C2more nearly equal (assuming L1 and L2 to be equal) the rejector effectstend to be merged into one another, and the acceptor resonance appearsonly as a small bump between rejector dips; for this reason, therefore,in practice the split, or branched, circuit arrangement described is notby itself very effective for the purpose in question.

Fig. 2 shows the effects of resistance. The curves of the said Fig.` 2are curves connecting relative impedances (ordinates) with frequency(abscissae) and illustrate the relative value of the impedances obtainedat the three resonance frequencies F1, Fo and F2 for different relativevalues of these resonant frequencies i. e., for vdifferent positions ofadjustment. It will be seen that there are four groups of curves shownin Fig. 2, each group being drawn to show the impedance values obtainedat a particular ratio fr 5 rn of F1 This ratio is marked in Fig. 2 by aand as will be seen there are four groups of curves one for wherea=1.01, one for where a=1.02, one for where a:=1.05 and one for wherea=1.1. For the sake of convenience, and in order not to have too manycurves on the same figure, each group of curves for the values a=1.02,a=1.05 is represented by one curve only there being ve curves in each ofthe two remaining groups.

The curves in the group for a=1.01 are indicated by the letters AI, BI,CI, DI, EI, respectively while the corresponding curves in the group fora=1.1 are indicated at A4, B4, C4, D4 and E4. The curve A2 is drawn forthe value a=1.02 and the curve A3 for the value a=1.05. The curves ineach group are drawn for different values of the reciprocal of the coildissipation factor i. e. are drawn for different values of where w=21rFand R is the resistance. The curves AI, A2, A3 and A4 represent the casewhere Q=100, the curves BI, B2, B3 and B4 represent the case whereQ=316, the curves CI, C2, C3 and C4 represent the case where 2:1000, thecurves DI, D2, D3 and D4 represent the case Where Q=3160 and the curvesCI, C2, D3 and C4 represent the case where Q=10,000.

It Will be apparent from Fig. 2 that for those cases where Q is of highvalue the resonance peaks and dips are very sharp while the generalshapes of the curves between those resonant peaks are not substantiallydilferent from the shapes of the curves obtained when Q is of low value,and it will therefore be seen that the general shapes of the curves aresuch that the distortion due thereto will be readily easily correctible.For normally goed coil employed at present day broadcasting frequenciesat one million cycles per second or thereabouts Q would have a value ofabout 100, and it will be obvious from Fig. 2 that the degree ofselectivity obtained from a branched circuit as described and havingcoils with a Q value of about will be quite insuiicient.

In carrying out the present feature of the invention, however, the Qvalue is greatly increased by applying negative resistance to the coilsand it will be seen that where Q is increased to the value of 3160(curves DI, D2, D3, D4) the peak to dip ratio is over 3000. With theapparatus normally available at the present time it will probably not beconvenient practically to raise the Q value much above 3000 to 40010though, of course, it is theoretically possible to do so.

Fig. 3 is a set of curves connecting relative impedances (ordinates)with frequency (abscissae), and shows the impedance for an ordinarytuned circuit with different values of coil dissipation, this tunedcircuit being tuned to 300 meters. The curve ABI is drawn for a Q valueof 100, E3I for a Q value of 316, C3I for .a Q value of 1000, D3! for aQ value of 31601 and E3I for a value of 10,000. Comparing the curves ofFig. 2 with those of Fig. 3 it will be seen that for a given value of Qthe ratio of impedance at Fn to that at F1 and F2 can be made very muchgreater than in the case of the ordinary circuit (illustrated by theFig. 3 now being referred to) because of the tWc fold effect whichenhances both peak and dips as the damping is reduced. Thus with thecircuit described any given impedance ratio necessary to reduce theinterference of a station whose frequency is F1 or F2 can be producedwith a lesser degree of applied negative resistance (which results ingreater circuit stability) and for the all important modulationfrequencies up to 5000 cycles less tone correction is required.

In the foregoing it has been assumed throughout that the radio frequencyresponse curves follow the impedance curves, and this is approximatelytrue so long as the circuits employed are connected across a circuit ofrelatively much higher impedance, e. g. the grid circuit of a valve.

The eect of connecting a circuit in accordance with this inventionacross a circuit of high impedance is mainly to blunt the peak occurringat Fa without much affecting the peaks occurring at F1 and F2, and byconnecting a circuit in accordance with this invention across a circuitof impedance of a relative value of about 10 the attening of the peak atFois such as substantially to simplify the problem of subsequent audiofrequency correction without much affecting the selectivity effect andby suitably .arranging shunt impedances it is therefore possible tocontrol the sharpness of the peak at F0 without substantially affectingthe sharpness of the dips at F1 and F2.

Fig. 4 shows a convenient arrangement in accordance with the firstfeature of this invention, and, as will be seen from the said Fig. 4,the 75 branched circuit consists of inductances L1 and Lz in series withcondensers C1 and Cz the conbringing the value Fo closer either to F1 orF2 according to the sign of the mutual inductance and thus producing anasymmetrical curve. If this effect is desired, as it may be in somecases, arrangements may be provided for controlling the mutualinductance between the coils. In the arrangement shown in Fig. 4 therequired negative resistance eifects are obtained by means of a pair ofscreened grid valves V1, V2 which are adjusted in manner known perse tooperate on the negative resistance portions of their characteristics.The amount of negative resistance injected may be controlled byAadjusting the potentials applied to the electrodes of the screened gridvalves.

As shown in Fig. 4, the two cathodes of the tubes V1 and V2 areconnected together and through a common connection the cathodes areconnected to'an intermediate point of thecurrent source. The anodes ofthe two tubes are also connected together through by means of acircuitwhich includes condensers C1 and Cz and also by a circuit whichincludes the two coils L1 and L2. A point intermediate the coils L1 andL2 is connected to a point of the source which is positive with respectto the cathode connection point thus maintaining the anodes positivewith respect to the associated cathodes. The outer grids of the twotubes are connected together and are maintained at a more positivepotentialA than the anodes by connection to a point of the source whichis more positive than theA anode connection point. The two inner gridsare preferably biased negatively and for this purpose provision is madeto connect each to a separate variable potentiometer device to allowseparate adjustment thereof. The potentiometer resistor in each case isshown connected across a portion of the source between the cathodeconnection point and a point of the source which is negative withrespect to the cathode connection point. l

It is not necessary that two screened grid valves be employed sinceobviously one valve can provide the negative resistance for bothbranches of the branched circuit, but the arrangement illustrated ispreferred since it is symmetrical. In order that the condensers CiC'zmay be. properly ganged together they should be so-called straight linefrequency condensers and by the use of correctly designed straight linefrequency condensers the whole circuit can be tuned i. e. moved up anddown the frequency scale while maintaining asubstantially constantfrequency separation between the dips and the peak (see Fig. 2) at allpoints. Y

This frequency separation will be dependent upon the difference betweenCi and C2 and a convenient arrangement therefor is to constitute thecondensers C1Ca by a pair of condensers having their rotors on the sameshaft and perfectly in line with one another while their stators are ata predetermined angle to one another. In fact by providing an adjustmentcontrolling the angular separation between the stators the frequencyseparation between F1 and F2 may be adjusted and a controllableselectivity effect thereby obtained. The circuit arrangement shown inFig. 4 may be employed in a variety of different ways, for example, asshown in Fig. 9 of the accompanying drawings in the case of an ordinaryreceiveran ordinary tuned circuit consisting of an inductance I shuntedby two condensers 2, 3, in series may be connected across the gridcircuit of the rst valve 4 of the receiver, the aerial 5 being tappedupon the inductance l, or, as shown, connected to the top thereof, andthe terminals H and E of an arrangement as shown in Fig. 4 beingconnected across one of the two condensers as' shown the condenser 3. GBis a grid bias source. By adjusting the condenser across which theterminals H and E are connected correct impedance matchingl may beobtained.

In a modication illustrated in Fig. 10 of the accompanying drawingsimpedance matching is obtained inductively. In this modification thetuned circuit consists simply of a coil la and shunt condenser 2aconnected across the grid circuit of the first valve 4 the aerial 5being tapped upon this inductance la, or as shown, connected to the topend thereof andthe terminal I-I being variably tapped also upon theinductance ia the terminal E being connected to earth. Similarly thearrangement of Fig. 4 may be associated with an inter-stage coupling,for example, as shown in the accompanying Fig. 11, the terminal H may beconnected to a variable tapping point upon the inductance Ib` in a tunedcircuit in the plate circuit of the first valve 6 of two valves 6 and 'lin cascade the terminal E being connected to the common cathode point(earth).

The accompanying Fig. 12 shows a slight modification of the arrangementshown in accompanying Fig. 11 the said modification consisting inreplacing the simple tuned circuit in the plate circuit of valve 6 inFig. l1 by a tuned circuit having two seriesI condensers 2b, 3by acrossan inductance Ib and connecting theterminal I-I to the point between thecondensers 2h and Eb instead of to a tapping point upon Ib. In anarrangement described wherein a selectivity device in accordance withthis invention is employed in conjunction with an ordinary tunedcircLL't (see the accompanying Figs. 9 and 10) the tuning operation maybe very simply employed in two stages the rst stage consisting in tuningthe ordinary tuned circuit to the station desired and the second stageconsisting in switching in the selectivity circuit and adjusting it toseparate out the interference.

For example, when, it is desired to improve the selectivity of a normalradio receiver for stations closely spaced in frequency the frequencyseparation can conveniently bearranged to be 9 kilocycles each side ofthe carrier Fo (this is the normal present'day separation betweenbroadcasting stations Whose frequencies are adjacent one another in thefrequency spectrum) and in this way, by the use of the selectivitycircuit, reception of a desired station can be eifeoted withoutinterference from a station of adjacent frequency. In cases where a highdegree of separation is not required it will in many cases not benecessary to provide any audio frequency correction circuit unless therequirements as to quality are very stringent and/ or the requiredselectivity very high a tone correction arrangement may be provided. Anyconvenient tone correction arrangement known per se may be employed forexample between the output terminals of the receiver and its associatedloud-speaker or in the audio frequency amplifier of the receiver.

Arrangements wherein a branched circuit' in accordance with the firstfeature of this invention is associated with and tapped upon a tunedcircuit, for example arrangements as shown in Figs. 9 to 12 of theaccompanying drawings, present the advantage that the combined effectsof the resonant tuned circuit and the branched circuit can be made tonullify to a great extent undesired effects due to the rising up of thecharacteristic on the side of the dip frequency remote from the peakfrequency. This type of advantageous result is illustrated graphicallyin the accompanying Fig. 13 in which the chain line curve is thecharacteristic of the tuned circuit alone, the broken line curve thecharacteristic of the branched circuit arrangement tapped thereon andthe full line curve the combined characteristic. As will be seenalthough in the full line curve characteristic rises outwardly fromthe.dips the amount of rise is substantially less than in the case ofthe broken line curve.l

Arrangements as hereinbefore described are well suited for use insuperheterodyne receivers and may with advantage be employed to impartto the intermediate frequency amplifier of a superheterodyne receiver adesired degree of selectivity. A superhetercdyne receiver embodyingapparatus as hereinbefore described is illustrated in the accompanyingFig. 14, the said gure only showing such parts of the receiver as arenecessary to an understanding of the manner in which the presentinvention is applied thereto. Referring to Fig. 14, 8 represents a highfrequency valve operating at the received frequency and having in itsoutput circuit the customary tuned circuit 9. The output from the valve8 is transferred to a first detector valve i0 to whose grid circuit isalso applied, as in the usual well known Way, local oscillations derivedfrom a local oscillator represented in Fig. 14 merely as a tuned circuitIl though, of course, there will either be provided a separateoscillator valve or the rst detector will be arranged to act both as amixing valve and an oscillator in known manner. l2 is the rst valve ofthe intermediate frequency amplier whose output is dealt with (byapparatus not shown) in the manner usual in superheterodyne receivers;

It will be noted that the plate circuit of the iirst detector valve l0includes a tuned circuit lb, 2b, 3b, resembling the correspondinglyindicated tuned circuit of Fig. 1l and that the junction point betweenthe condensers 2b and 3b is connected to the terminal H of a branchedcircuit arrangement closely resembling that shown in Fig. 4 of thedrawings. This branched circuit arrangement includes negative resistancevalves V1 condensers C1C2 and inductances L1L2 all connected after themanner o-f the said Fig. 4. In addition, however, there are provided twosmall auxiliary condensers k1 and k2 which are mechanically coupledtogether so that when one is at its maximum value the other is at itsminimum value.

By adjusting the condensers kika together itis thus rendered possible tovary the frequencies at which the dips occur without changing the peakfrequency in order to meet any specific requirement at any time.Assumingthe intermediate frequency to be 110,000 cycles per second, ithas been found possible to adjust the apparatus between extremepositions at one of which (that .corresponding to extreme selectivity)the fdip frequencies occurv at 3,000 cycles above and below the peakfrequency and at the other of which (that inv which minimum selectivityis obtained) the dip frequencies occur at 20,000 cycles above and belowthe peak frequency. In Fig. 14 it is assumedthat the xed condenser C2 issmaller than the xed condenser C1 and the maximum value ofthe auxiliarycondenser k1 smaller than that of the auxiliary condenser k2.

With this arrangement greatest selectivity will be' obtained whentheauxiliary condenser k2 is at maximum value and-the auxiliarycondenser Ici at minimum value. Where adjustment of selectivity-isnot'required and a fixed predetermined selectivity is all that 'isnecessary the auxiliary condensers k1 and 'k2 lmay of course be omitted.Apparatus in accordance with this invention can also be employed inconnection with superheterodyne receivers'to` provide a high frequencytuning arrangement whereby what is commonly known as second channelinterference may be successfully eliminated. In such an application anarrangement as shown in Fig. 4 would be employed and would be soadjusted that the dip frequencies were remote from the peak frequency byan amount vequal to twice the intermediate frequency.

Arrangements in accordance with the first feature of this inventionpossess a characteristic which may, in some cases, be undesirable inpractice. For example when a circuit as illustrated in Fig. 4 isemployed in a radio Vreceiver whose other selective circuits arerelatively flatly tuned, it will be found that owing to the fact thatthe response curve rises outwardly of the rejector dips, a condition mayarise when a comparatively strong signal emitted at a frequencyrelatively remote from that intended rto be received may causeinterference. For example, the radio broadcast stations known as LondonRegional and Muhlacker operate at the present time on closely adjacentfrequencies (only about 9 kilocycles apart) and if a circuit arrangementas illustrated in Fig. 4 were adjusted to cut out the London Regionalstation, and receive relatively weak signals from Muhlacker (the radioreceiver being assumed to be much nearer to the London Regional stationthan to Muhlacker) then, owing to the risingv response above referred toand to the low selectivityof the circuits of the receiver following thesaid circuit as illustrated in Fig. 4,

a comparatively strong signal from a station such as the MidlandRegional station relatively remote in frequency from that of Muhlackermay cause interference.

'The second feature of this invention, which obviates this difficulty,will be better understood upon reference to the diagrammatic andexplanatory graphical illustrations shown in Figsf', 6, 7. Consider'thecase of a complex split or branched circuitv arrangement consisting of acapacity shunted by'- an inductance and a capacity in series and supposethe acceptor circuit constituted by the inductance and capacity inseries has a. natural frequency of 990,000 cycles per second, the wholeloop-circuit having a natural frequency of '1,000,000 cycles per second.

Suppose the Q value of this circuit to be 1,000. Then the curveconnecting the relative impedance of the whole-circuit with frequencywill be as shown in Figk. Since in this circuit arrangement the rejectorpath will behave as an infing the damping.

acceptor effect and the condenser in shunt with the series inductanceand capacity of course constitutes this tuning condenser. Similarlyconsider'the case of an inductance shunted by an inductance and acapacity in series, the series connected inductance and capacity beingtuned to 1,010,000 cycles per second and the whole loop circuit beingtuned to 1,000,000 cycles per second, Q being, as before, 1,000.

Now the curve connecting relative impedance with frequency will be asshown in Fig. 6 in which the rejector or dip effect is obtained at1,010,000 cycles per second, an inductance is required to attain anover-all acceptor effect. If in accordance with the present feature ofthe invention the two complex split or branched circuits just describedbe employed in cascaded association the total resultant response effectwill be of the general form shown by the series of curves of Fig. 7.This yseries of curves consists, as will be seen, of three curves shownrespectively in full broken and chain lines. The full line curve isdrawn for the case where the capacity tuned circuit rejects 990,000cycles per second and accepts 1,000,000 cycles per second the inductancetuned circuit accepting the same frequency, namely 1,000,000 cycles persecond but rejecting 1,010,000 cycles per second.

The broken line curve is drawn for the case in which the condenser tunedcircuit rejects 987,500 cycles per second and accepts 997,500 cycles persecond the inductance tuned circuit accepting 1,002,500 cycles persecond and rejecting 1,012,500 cycles per second. The remaining chainline curve is drawn for the case where the capacity tuned circuitrejects 992,500 cycles per second and accepts 1,002,500 cycles persecond the inductance tuned circuit accepting 997,500 cycles per secondand rejecting 1,007,500 cycles per second.

It will be apparent from a consideration of Fig. 7 and a comparison ofthis figure with the graphs of Fig. 3 that the result of cascading thecircuits and staggering the tuning as above set forth, is to provide aband pass effect with very steep sides to the response curve, theresponse remaining low on either side of the dips between which the bandpass response is obtained.

Fig. 8 of the drawings shows one cascade circuit arrangement inaccordance with the present feature of the invention. As will be seenupon reference to Fig. 8 of the drawings, two split or branched complextuned circuit arrangements are employed one, marked A, being a capacitytuned circuit and the other, marked B, being an inductance tunedcircuit. These two circuits are ganged together at a constant frequencyseparation for simultaneous control and thermionic valves constituted byscreened grid valves adjusted to present negative resistance areassociated with the two branched circuits for reduc- One of thesenegative resistance valves is indicated at V1 and the other at V2. Inputis applied between terminal Iv and earth (cathode point) and outputtaken off between terminal O and earth (cathode point). It will be notedthat the two branched or split circuits are in effect cascaded via whatmay be termed an isolating valve which is the valve V3 of Fig. 8, thisvalve being also represented as a screened grid valve.

This arrangement is provided because it is important that the twocascaded circuits upon which the arrangement depends for its selectivityshould be adequately screened from one an- 76 other, and in carrying thecircuit arrangement of the said Fig. 8 into practice, care should betaken to provide adequate screening and toisolate the various energysupplies for the valves. Such isolation may be effected in any mannerknown per se e. g. as indicated by the screens represented in the saidFig. 8 by broken lines. It will be appreciated by those skilled in theart that gang control is possible, for, if all the condensers ofcircuits A and B be ganged together and with the mutual inductance,their individual laws of variation being correctly chosen any of thecurves of Fig. 7 could be reproduced at any wave length within therange. In practice, however, such an arrangement would probably beunduly clumsy and a compromise sufficient for most practical purposes isto be preferred. Such a compromise consists in leaving the acceptorcondenser in circuit A and the parallel inductance in circuit B iixedand making the two rejector condensers (i. d. those in series with theinductance elements) of such individual laws that they could be gangedto produce a constant band-width in kilocycles at all frequencies withinthe range.

It would be a relatively simple matter to reproduce these laws fromstandard straight line frequency condensers ganged on one shaft.However, if correct adjustments of the parallel condenser in A andparallel inductance in B were made so as to produce a curve'entirelysymmetrical in the middle of the tuning range then, at one end of therange, although the correct band width at the top of the curve would bemaintained, one side would be steepened and the other made less steep.At the other end of the range the effect would be inverted, i. e. againthe width at the top would be correct, but the side which was thesharpest' at the other extreme end of the rangewould not be the leastsharp, and vice versa. f

In other words, the compromise arrangement, would suffer from the defectthat the response curve would only be exactly symmetrical in the centerof the tuning range. This, however, would not be a too serious objectionin many practical cases.

Although the present invention is of general application it may,'withconsiderable advantage, be employed as a xed tuned arrangement in theintermediate frequency tuning stage of ak superheterodyne receiver. Aportion of an amplifier embodying what may be regarded as a developmentof the arrangement illustrated in Fig. 8 is illustrated in theaccompanying Fig. 15. Referring to Fig. 15 the valve Illa which may forexample be a high frequency valve in a radio receiver or intermediatefrequency valve in a superheterodyne receiver, includes in its outputcircuit a tuned circuit Ib, 2b and 3by corresponding to the similarlydesignated tuned circuit shown in the accompanying Fig. `11.' Thecondenser 3b forms part of a branched circuit arrangement similar tothat shown at A in Fig. 8, there being provided a reaction valve Vi asin the said Fig. 8. In the accompanying Fig. l5, V3 is an isolatingvalve corresponding to the similarly designated valve of the Fig. 8,this isolating valve containing in its output circuit a simple tunedcircuit includingan inductance lb part of which constitutes theinductive branch of a circuit arrangement B corresponding to thesimilarly designatedcircuit arrangement of Fig. 8, there being provideda further negative resistance valve V2 as in the said Fig. 8. Theprincipal diiferences between the accompanying Fig. 15 and Fig. 8, arethat in the said Fig. 15 the circuits A and B are in effect tapped upontuned circuits; that. parts of the reactances of the tuned circuits alsoconstitute parts of the reactances of the circuits A and B; and that thesaid circuits A and B contain blocking condensers K of large value andwhich may, so far as the working frequencies are concerned, be regardedas though they were short circuited.

The simplified tapped circuit arrangement embodied in Fig. 15 is such asto render it possible to dispense with the isolating valve V3 bysuitably adjusting or selecting the tappings.

What I claim is:

l. In combination with a resonant circuit tuned to a desired carrierfrequency of a modulated carrier wave, a frequency selective network ofthe band pass type comprising at least two branch circuits, eachincluding an inductor and a capacity element, one of said branchcircuits being tuned slightly below the desired frequency and the otherthereof being tuned slightly above the desired frequency, and a negativeresistance means connected to said branch circuits for reducing thedamping of the selective network whereby a predetermined selectivitycharacteristic is imparted to the resonant circuit, said frequencyselective network being connected across a portion of said resonantcircuit.

2. In combination with a resonant circuit tuned to a desired carrierfrequency, a frequency selective network of the band pass type,comprising at least two branch circuits, each including an inductor anda capacity element, one of said branch circuits being tuned slightlybelow the desired frequency and the other branch circuit being tunedslightly above the desired frequency, a negative resistance meansconnected to said branch circuits for reducing the damping of theselective network whereby a predetermined selectivity characteristic isimparted to the resonant circuit, the inductor and pacity element ineach branch circuit beingv in series and'said resistance means includingat least one screen grid tube connected to operate in dynatron fashion.

3. In combination with a resonant circuit tuned to a desired carrierfrequency of a modulated carrier wave, a frequency selective network ofthe band pass type comprising at least two branch circuits, eachincluding an inductor and a capacity element, one of said branchcircuits being tuned slightly below the desired frequency and the otherbranch circuit being tuned slightly above the desired frequency, anegative resistance means connected to said branch-circuits for reducingthe damping of the selective Ynetwork whereby a predeterminedselectivity characteristic is imparted to the resonant circuit, eachcapacity element being variable, said resonant circuit including atleast two reactances of opposite sign, one of the reactances beingconnected to at least one of said branch circuits.

4. In combination with a tunable resonant circuit, a frequency selectivenetwork of the band pass type comprising at least two branch circuits,each including an inductor and a capacity element, one of said branchcircuits being tuned to a frequency which is slightly below thefrequency to which the tunable resonant circuit is tuned and the otherbranch circuit being tuned to a frequency which is slightly above thefrequency to which the tunable resonant circuit is tuned, and negativeresistance means comprising a pair of screen grid tubes connected tooperate as dynatrons, said negative resistance means being connected tosaid branch circuits for reducing the damping of the selective networkwhereby a predetermined selectivity characteristic is imparted to theresonant circuit.

5. In combination with a resonant circuit tuned to a desired carrierfrequency of a modulated car rier wave, an amplifier tube connected to aresonant circuit, a frequency selective network of the band pass typecomprising at least two branch circuits, each including an inductor anda capacity element, one of Said branch circuits being tuned slightlybelow the desired frequency and the other thereof being tuned slightlyabove the desired carrier frequency, and -a negative resistance meansconnected to said branch circuits for reducing the damping of theselective network whereby a predetermined selectivity characteristic isimparted to the resonant circuit, said selective network including anadjustable capacity element 'across each of the branch circuit capacityelements.

6. In a receiver of the Superhetercdyne type including a frequencychanger tube and a following intermediate frequency ampliner tube, atuned circuit including an inductance and at least one condenserconnected between the. two tubes, a frequency selective network coupledto said tuned circuit, said network comprising a pair of coupled tunedbranch circuits, one of said branch circuits being tuned to a frequencywhich is slightly below the frequency to which the tuned circuit istuned and the other thereof being tuned to a frequency which is slightlyabove the frequency to which the tuned circuit is tuned and negativeresistance means connected to said branch circuits.

7. In a receiver of the superheterodyne type including a frequencychanger tube and a following intermediate frequency ampliiier tube, atuned circuit including an inductance and at least one condenserconnected between the tubes, a frequency selective network -coupled tosaid tuned circuit, said network comprising a pair of coupled tunedbranch circuits each of said branch circuits comprising inductance yandcapacity in series, one of said branch circuits being tuned to afrequency which is slightly below the frequency tol which the tunedcircuit is tuned and the other branch circuit being tuned to a frequencywhich is slightly above the frequency to which the tuned circuit istuned, an adjustable condenser connected across at least one of thecapacities and negative resistance means connected tosaid branchcircuits.

8. In a receiver of they superheterodyne type including a frequencychanger tube and a following intermediate frequency vamplifier tube, atuned circuit including an inductance, and at least one condenserconnected between the tubes, a frequency selective network coupled tosaid tuned circuit, said network comprising a pair of coupled tunedbranch circuits, one of said branch circuits being tuned to a frequencywhich is slightly below the frequency to which the tuned circuit istuned and the other branch circuit being tuned to a frequency which isslightly above the frequency to which the tuned circuit is tuned, andnegative resistance means comprising a pair of dynatron tube circuits,said lastnamed means being connected to said branch circuits.

9. In a receiver of the superheterodyne type including a frequencychanger tube and a following intermediate frequency amplier tube, atuned circuit including an inductance and at least one condenserconnected between the two tubes, an auxiliary condenser in series withsaid condenser, a frequency selective network coupled to said tunedcircuit, said network comprising a pair of coupled tuned branchcircuits, one of said branch circuits being tuned to a frequency whichis slightly below the frequency to which the tuned circuit is tuned andthe other branch circuit being tuned to a frequency which is slightlyabove the frequency to which the tuned circuit is tuned, said selectivenetwork being connected across the auX- iliary condenser and negativeresistance means connected to said branch circuits.

10. In combination with a pair vof cascaded am'- plifier tubes, atunable circuit connected to the output electrodes of the rst tube, thecircuit including a coil and a variable tuning condenser, a frequencyselective network coupled to said tunable circuit and including a branchcircuit consisting of means comprising an inductance and capacity inseries for tuning the branch circuit to a frequency which is justoutside the frequency to which the tunable circuit is tuned, said branchcircuit being connected to one of said reactive elements of the tunablecircuit, and negative resistance means connected to said network toreduce the damping thereof.

11. A frequency selective network comprising a pair of branch circuits,each circuit including a coil and Variable condenser in series, saidbranch circuits being tuned to slightly different frequencies and anegative resistance element connected in shunt with each branch circuit,a pair of screen grid tubes, a source of direct Voltage for the tubes, acommon connection from a point on said source to one side of each of thecoils, the anode of each tube being connected to the opposite side ofone coil and means for connecting the screen grids of said tubes to apoint on said source which is positive with respect to the voltage ofthe anodes.

12. A frequency selective network comprising a pair of branch circuits,each circuit including a coil and a Variable condenser in series, saidbranch circuits being tuned to slightly different frequencies, acondenser connected in shunt with one branch circuit, a coil connectedacross the other branch circuit, a pair of screen grid tubes, a sourceof direct voltage for the tubes, a common connection from a point onsaid source to one side of each of the coils, the anode of each tubebeing connected to an opposite side of one coil, and means forconnecting the screen grids of said tubes to a point on said sourcewhich is positive with respect to the voltage of the anodes.

13. In combination with a resonant circuit tuned to a desired carrierfrequency of a modulated carrier wave, a frequency selective network ofthe band pass type comprising at least two branch circuits eachincluding an inductor and a capacity element, one of said branchcircuits being tuned slightly below the desired frequency and the otherthereof being tuned slightly above the desired frequency, and a negativeresistance means connected to said branch circuits for reducing thedamping of the selective network whereby a predetermined selectivitycharacteristic is imparted to the resonant circuit, said resonantcircuit comprising an inductance deviceand a pair of tuning condensersin series shunted across the inductance device, said frequency selectivenetwork being connected across one of said last named condensers.

14. In a superheterodyne receiver an amplifier tube provided with aninput circuit and an output circuit, a source of signalling energy ofpredetermined frequency, means for coupling said source to said inputcircuit, a utilizing circuit and means for coupling the utilizingcircuit to said output circuit, said means for coupling the source tothe input circuit comprising a tunable circuit tuned to a frequencywhich is slightly different than the frequency of the energy to beamplified, said means for coupling the output circuit to the utilizingcircuit comprising a tunable circuit tuned to a frequency which isslightly diierent than the frequency of the energy to be amplied but inan opposite direction than the frequency to which said first named meansis tuned and means connected with the coupling means for providing highattenuation in said circuits at the frequencies to which the couplingmeans are tuned.

NoL MEYER RUsT.

